1. Field of the Invention
The present invention relates to an apparatus and a method for transmitting and receiving ultrasonic waves having an acoustoelectric, ultrasonic transducer.
2. Description of Related Art
An ultrasonic transducer converts ultrasonic waves into an electric signal. A piezoelectric, ultrasonic transducer includes a piezoelectric element, and the piezoelectric element produces an electric signal when subjected to pressure or strain induced by ultrasonic waves. The electric signal converted by the piezoelectricity is sensitive to phase of ultrasonic waves. The electric signal is alternating, and its frequency depends on that of ultrasonic waves.
Recently, acoustoelectric, ultrasonic transducers have received attention. Some piezoelectric semiconductors are acoustoelectric, converting ultrasonic waves into an electric signal accompanied by phonon-charge carrier interaction. In contrast to the piezoelectric transducer, the electric signal converted by the acoustoelectricity is insensitive to phase of the ultrasonic waves, and the electric signal is not alternating.
Ultrasonic transducers are used in a variety of non-destructive tests. The acoustoelectric, ultrasonic transducer is superior to the piezoelectric, ultrasonic transducer in some applications where ultrasonic waves received by the ultrasonic transducer have phase distortion. In such applications, piezoelectric, ultrasonic transducers tend to be erroneous whereas acoustoelectric, ultrasonic transducers are accurate, not affected by the phase distortion. This is the case, for example, in measuring a thickness of a sample having a cylindrical shape, and a rough radial surface and in detecting defects inside a composite material.
However, in reality, acoustoelectric, ultrasonic transducers do not give only an acoustoelectric signal, but a mixture of an acoustoelectric signal and a piezoelectric signal, wherein an acoustoelectric signal is insensitive to the phase of ultrasonic waves received by the transducer, and a piezoelectric signal is sensitive to the phase of ultrasonic waves received by the transducer. Therefore, it has been a problem in acoustoelectric, ultrasonic transducers to isolate the acoustoelectric signal from the piezoelectric signal.
Japanese Patent Application Laid-Open No. 5-72579 discloses an apparatus having a transmitter for sending ultrasonic waves into a sample, an acoustoelectric, ultrasonic transducer for receiving ultrasonic waves reflected by the sample, and an electric frequency filter. The electric frequency filter removes the piezoelectric signal in the output signal of the acoustoelectric, ultrasonic transducer so as to give the acoustoelectric signal. The piezoelectric signal has a higher frequency corresponding to ultrasonic waves whereas the acoustoelectric signal has a lower frequency. Therefore, the electric frequency filter removes the electric signal having a higher frequency, thereby selecting the acoustoelectric signal having lower frequency.
However, as an energy of ultrasonic waves received by the acoustoelectric, ultrasonic transducer decreases, the acoustoelectric, ultrasonic transducer gives a smaller ratio of the acoustoelectric signal and a larger ratio of the piezoelectric signal. The acoustoelectric signal is proportional to the energy of ultrasonic waves whereas the piezoelectric signal is proportional to the square root of the energy of ultrasonic waves. When ultrasonic waves have a small energy, the electric frequency filter may not sufficiently remove the large piezoelectric signal, compared to the small acoustoelectric signal.
Moreover, in the apparatus of Japanese Patent Application Laid-Open No. 5-72579, which belongs to a so-called pulse-echo type, the acoustoelectric, ultrasonic transducer for receiving ultrasonic waves has a thickness several times greater than the wavelength of the ultrasonic waves so as to increase the difference in frequency between the acoustoelectric signal and the piezoelectric signal. The acoustoelectric signal has a longer duration time, decreasing time resolution thereof.
Japanese Patent Application No. 6-59093 discloses an acoustoelectric, ultrasonic transducer having an interference layer thereon so as to give a time lag to a portion of ultrasonic waves received by the acoustoelectric, ultrasonic transducer, thereby reducing the piezoelectric signal. Typically, the portion of the ultrasonic waves delays by a half the period thereof whereas the other portion of the ultrasonic waves do not have any delay, thereby offsetting the phase of each other. The acoustoelectric, ultrasonic transducer can selectively detect the acoustoelectric signal even from ultrasonic waves having a limited energy.
However, the decrease in the piezoelectric signal by the interference layer depends on the frequency of the ultrasonic waves received. Therefore, where ultrasonic waves having a mixture of frequencies are received by the acoustoelectric, ultrasonic transducer, the transducer needs a plurality of the interference layers corresponding to the frequencies so as to reduce the piezoelectric signal.
The ultrasonic waves may need to have homogeneous sound field so as to sufficiently decrease the piezoelectric signal by the interference layer. The interference layer has a receiving surface for receiving ultrasonic waves, and the receiving surface may not have a shape complementary to the sound field of the ultrasonic waves. For example, the ultrasonic waves may strike the receiving surface with a small angle; ultrasonic waves may have a time lag in striking the interference layer; and ultrasonic waves may have intensity modulation. In these cases, the piezoelectric signal may not sufficiently decrease.
In the pulse-echo method, typically, an ultrasonic wave having a duration of one to several periods is transmitted to a sample, and the interference layer cannot decrease the piezoelectric signal of the first half period and the last half period of the ultrasonic wave.